[Mythically, from the Latin semi-deponent verb quuxo, quuxare,
quuxandum iri; noun form variously ‘quux’ (plural
‘quuces’, anglicized to ‘quuxes’) and
‘quuxu’ (genitive plural is ‘quuxuum’, for four
u-letters out of seven in all, using up all the ‘u’ letters in
Scrabble).]

1. Originally, a metasyntactic variable like
foo and foobar. Invented by
Guy Steele for precisely this purpose when he was young and naive and not
yet interacting with the real computing community. Many people invent such
words; this one seems simply to have been lucky enough to have spread a
little. In an eloquent display of poetic justice, it has returned to the
originator in the form of a nickname.

2. interj. See
foo; however, denotes very little disgust, and is
uttered mostly for the sake of the sound of it.

3. Guy Steele in his persona as ‘The Great Quux’, which
is somewhat infamous for light verse and for the ‘Crunchly’
cartoons.

4. In some circles, used as a punning opposite of
‘crux’. “Ah, that's the quux of the matter!”
implies that the point is not crucial (compare
tip of the ice-cube).